* Four former Credit Suisse traders expected to be charged -sources
* Credit Suisse cooperating with investigators -sources
* Charges stem from financial crisis (Adds details on those charged)
By Lauren LaCapra and Jennifer Ablan
Jan 31 (Reuters) - U.S. authorities are preparing to charge four former Credit Suisse Group AG employees with criminal and civil fraud related to write-downs on subprime mortgage derivatives at the height of the financial crisis, sources familiar with the matter said.
Credit Suisse will not be charged in the matter, which is being investigated by federal prosecutors and the U.S. Securities and Exchange Commission, the sources said.
The four people to be charged were former Credit Suisse traders who were fired, another source said, but it was unclear when and for what reason. The suspected illegal conduct took place roughly four years ago, the source said, adding that the bank had been cooperating with officials.
The investigation stems from $2.85 billion in write-downs that Credit Suisse took on collateralized debt obligations in 2008, said the sources, who spoke on the condition of anonymity.
Credit Suisse revealed those CDO losses in early 2008, and blamed them on a group of rogue traders - who the bank said had deliberately mispriced securities - and on a failure of internal controls.
Credit Suisse, the Federal Bureau of Investigation, the SEC and Manhattan U.S. attorney Preet Bharara declined to comment on the matter.
Charges could come as early as Wednesday, people familiar with the matter said, but the timing was uncertain.
The planned charges come as the Obama administration is stepping up efforts to prosecutes Wall Street bankers and others for misconduct related to the financial crisis. Last week during his State of the Union address, President Obama announced the formation of a mortgage-fraud task force to pursue subprime-related cases.
The collapse of the subprime housing market was one of the triggers of the worst financial crisis since the Great Depression. (Additional reporting by Matthew Goldstein, Basil Katz and Sarah N. Lynch; Editing by Gary Hill)